Numerous developmental studies have detected very early behavioral and biological changes in infants and toddlers at high-risk for depression. Despite these highly suggestive findings, prior to the investigator's preliminary study, there was insufficient data to address the question of whether children younger than six could manifest clinically significant depression. The need to identify depression at the earliest developmental point is underscored by the known chronic and relapsing course, and relative treatment resistance of the disorder when identified after six years of age. The investigator and colleagues are completing a federally funded study of the nosology of preschool depression. Findings from this study are the first to demonstrate that a valid depressive syndrome can be identified in preschool children using DSM-IV MDD criteria when the assessment is modified for age adjusted symptom manifestations. Preliminary validation of this syndrome has been established based on the association with numerous markers including a specific and stable symptom constellation, increased family history of related disorders, significant depression severity compared to controls and social impairment. Study findings raised questions about the duration of an episode of clinical depression and the criteria that distinguish syndromal from sub-syndromal states. This application proposes to utilize the sensitive and specific criteria derived from this study to ascertain a community based sample of depressed preschoolers (and appropriate controls) for a field trial and longitudinal follow-up. To address questions raised by the preliminary study, this investigation proposes to define the clinical characteristics of preschool depression using a new comprehensive and age appropriate diagnostic measure, in addition to assessing impairment in multiple domains. The need to validate diagnostic criteria for preschool depression is heightened by the alarming national increase in the off-label prescription of antidepressants to these young children, representing a serious public health crisis. While treatment unsupported by data is an obvious danger, the potential for earlier intervention in depression is a compelling possibility that must be aggressively explored. Valid diagnostic criteria are necessary for these future treatment studies and to facilitate investigations of the developmental neurobiology of this disorder.